Self stabilizing radiant tube burner



y 1965 J. D. NESBlTT ETAL SELF suamzme RADIANT TUBE BURNER Filed Nov. 28 1960 2 Sheets-Sheet 1 I INVENTORS: Jamar l7 NESBITT.

HERBERT M. Knmz- AT T 1.

July 20, 1965 SELF STABILIZING RADIANT Filed NOV. 28. 1960 J. D. NESBITT ETAL TUBE BURNER 2 Sheets-Sheet 2 35' K mvmvroas: 44 Jmm D.NESBITT.

4 g BY HERBERTMKBHN.

ATTE'.

United States Patent 3,195,609 SELF STABlL-lZlNG RADIANT TUBE BURNER John D. Nesbitt and Herbert M. Kohn, Tole-do, Ohio, assignors to Midland-Ross Corporation, Toledo, Ohio, a corporation of Ohio Filed Nov. 28, 1960, Ser. No. 72,221 7 (Jlaims. (Cl. 15S--7) This invention relates to a burner. More particularly this invention relates to a self-stabilizing burner for etfecting combustion according to the principle of diffusion combustion within a radiant heating tube which is maintained at a slightly sub-atmospheric pressure.

Radiant tube combustion may be accomplished either according to the principle of pro-mix combustion or according to the principle of diffusion combustion. Pre-mix combustion is a combustion technique in which the stream of fuel is completely aerated with combustion supporting air before it is combusted within the radiant tube. Diffusion combustion, on the other hand, is a combustion technique in which the stream of fuel is combusted as it gradually mixes or dilfuses with a separate concurrently flowing stream of air. The burners for effecting these combustion techniques are respectively classified as premix burners and diffusion burners.

The principle of radiant tube difiusion combustion may further be considered to be sub-divided into the sub-principles of laminar flow diffusion combustion and turbulent flow difiusion combustion Laminar flow diffusion combustion is a diffusion combustion technique in which the Reynolds number of the separately flowing streams of fuel and air within the radiant tube lies within the laminar flow region. When laminar flow diffusion combustion is employed the intermixing of the streams of 'fuel and air will occur very slowly and the resulting combustion reaction will be characterized by a long flame with relatively low radiant tube heat release rates (as expressed in Btu. per hour per square foot of radiant tube surface). Turbulent ilow diffusion combustion, on the other hand, is a diffusion combustion technique in which the Reynolds number of the fuel and air streams within the radiant tube lies within the turbulent flow region. When this combustion technique is employed the intermixing of the streams of fuel and air will occur fairly rapidly because the natural diffusion that occurs between the streams will be supplemented by turbulent intermixing. This combustion technique is characterized by a somewhat shorter flame than the flame length which is characteristic of laminar flow diffusion combustion. Also, the radiant tube heat release rates are somewhat higher when turbulent flow diffusion combustion is employed than they are when laminar flow diffusion combustion is employed.

Of the two (2) general radiant tube combustion techniques, e.g., pro-mix combustion and diffusion combustion, the latter technique has long been preferred by prior artisans as may be evidenced by referring to a long line of United States patents including 2,047,471 to Hepburn et al. 2,051,099 to Munford, 2,148,466 to Hepburn et al., 2,843,107 to Cipriani et al. and 2,873,798 to Knight. The selection as between laminar flow diffusion combustion and turbulent flow diffusion combustion has ordinarily been governed by economic conditions.

When radiant tube firing was first widely introduced in the 1930s the commercial fuel most prevalent was manufactured gas, the so-called Town-Gas. This fuel source was relatively expensive and it was therefore im portant to attain maximum heat recovery in a radaint tube furnace. Because the lower heat release rates characteristic of laminar flow diffusion combustion made it possible to operate the associated radiant tube at a lower thermal head and thereby achieve more efficient heat recovery it "ice Was the principle of laminar flow diffusion combustion which first gained wide acceptance in the trade. Hepburn et al. Patent 2,047,471 describes a burner of that era which was highly suited for this combustion technique.

In more recent times, however, the requirement for efficient heat recovery has become less critical due to the widespread introduction of natural gas, a less expensive fuel. During this same period there has arisen a strong demand for higher radiant tube heat release rates to reduce the number of expensive heat resistant alloy radiant tubes per furnace. To meet these requirements the prior artisans turned to the principle of turbulent flow diffusion combustion. Prior art burners, such as the aforesaid burner of Hepburn et al. 2,047,471, were not suited for the newer firing technique because the increased rates of flow of fuel and air greatly aggravated the problem of flame instability which has heretofore characterized all modes of radiant tube diffusion combustion. To meet this and other problems the burner of Knight 2,873,798 was developed. This burner utilized as a flame stabilizing technique, a cluster of pre-mix pilots adapted to provide a very high degree of pro-mix piloting to offset the poor flame stability of a turbulent flow diffuser burner. Another flame stabilizing technique adopted by some prior artisans was the partial pro-mix technique which comprised mixing the fuel supply to the radiant tube with a portion of its combustion supporting air. Other prior artisans used both pro-mix piloting plus partial pro-mixing of the fuel stream to achieve adequate flame stability in turbulent flow diffusion combustion.

The adoption of these expediencies of heavy pro-mix piloting and/ or partial pro-mixing of fuel supply as substitutes for flame stability have given rise to other inherent disadvantages. First these expediencies substantially increase the structural complexity of the burner itself and of the adjoining piping and manifoldingsystems. Secondly the application of pro-mix combustion to a radiant tube, either in the form of pro-mix piloting or in the form of partially premixing the main gas stream, tends to cause overheating in the firing end of the radiant tube. This problem is especially acute in high temperature furnaces where a relatively long portion of the firing end of the radiant tube is surrounded or shielded by the thick insulating refractory wall of the associated furnace. The severity of this problem gave birth to the burner of Ci-priani et al.

2,843,107, an extremely complex burner devised primarily to retard combustion within the shielded portion of the firing end of a radiant tube.

Third, the use of pre-mix piloting greatly impairs the changeover of the furnace to standby fuels. In steel mill furnace applications, for example, where the burners frequently are alternately operated on coke-oven gas and natural gas, it is necessary to change the mixing orifices in the pre-mix mixing system for each fuel conversion. In some installations it has even been found necessary to change the pre-mix pilot burners to achieve satisfactory operation on standby fuels. Obviously, these are time consuming and expensive chores. Also, in this regard, it is to be noted that the small mixing orifices required in a pre-mix mixing system are easily clogged by impurities usually present in coke-oven gas.

A fourth disadvantage of prior art burners is they do not readily lend themselves to throttling type of firing control which is more desirable than on-olf control because it minimizes the fatiguing effect of cyclic thermal expansion and contraction. Partial pro-mix firing in proportion to the degree of pro-mixing resorted to, involves the problem of backfire at low firing rates common to conventional pro-mix combustion. This problem can be overcome only by limiting the turndown range of such burners to a range that is usually too narrow for proper throttling control. Heavily piloted burners, on the other E3 hand, such as the aforesaid Knight burner, usually have such a high degree of pilot input that elfective throttling turndown would require that the pilot input also be throttled. This complicates the control system quite substantially and is seldom utilized;

It is, therefore, the objectof our invention to provide a diffusion type radiant tube burner that is self-stabilizing. It is a further object'of our invention to provide a radiant tube burner of simplified construction'yet capable of overcoming the'problem of overheating the shielded portion of the firing leg of a radiant tube. The heart of this burner is'a combustioncup located in fiuid communication with the firing endof' a radiant heating tube and'directed co-axially therewith. A stable flame front maybe maintained at a point in the combustion cup at all'firing rates without the use of separate pilots or partial premix in. thefollowing manner. Fuel for combustion is admitted axially to the combustion cup. A fuel conduit extension is provided within the cup, the aforesaid extension terminating within the cup at a point somewhat removed from the backplate of the cup. A stream-of air is admitted tangentially to the cup at a point upstream of the point'at which the fuel conduit extension terminates within the cup. The tangentially'admitted air stream is caused to spin about the fuel conduit extension forming a vortex adjacent the termination of the exten, sion. The slight suction caused. by the vortex tends to inspirate a portion of the fuel stream andcauses itto flow, in reservrse, towardthe backplate. This portion of the fuel streamand the spinning air stream tend to mix by virtue of the turbulence of the movements. These streams will,.when the variables are properly adjusted as will be explained later, tend to form a'highly stable combustible mixture at the combustion cup backplate. This combustible mixture, once ignited, acts as a piloting system for the'subsequent combustion of the remaining portion of the fuel stream.

The remaining portion of the fuel stream, which is' the much largerportion, is burned within the radiant tube in a somewhat conventional diffusive manner. A secondary air stream is admitted to the radiant tube through an annularv opening defined by the radiant tube or an extension thereto, and the combustion cup; The secondary. air stream surrounds the partially. burned fuel stream and flows, in effect, generally parallel. thereto whereby the streams ultimately mix. by a combination of turbulent mixing and natural diffusion.

To combat the problem of. overheating the firing leg of the radiant tube in the region where it is shielded by the insulating'refractory which-lines the furnace wall, a series of turning vanes are inserted in the annular secondary air inlet. These turningvanes impart a spinning motionto the annular. secondary air stream which retards mixing of the air and fuel streams. Inretarding the mixing of the fuel and air streams by means of turning vanes the problem of overheating the firing leg is combatted in two ways. First, the amount of heat released in this region is decreased. Second, the rapid motion of the spinning, air stream tends to'wipeheatolf the-wallof the radiant tube by convection thereby tending to positively cool the shielded-portion ofthe firing leg.

For a further consideration of what we consider to be novel and our invention, attention is directed to'the preferred form of our invention asdisclosed in the following portion of the specification, the appended claims, and the accompanying drawing.

In the drawing: I

FIGURE 1 illustrates an embodiment of a burner embodying the invention.

FIGURE 2 illustrates isometrically a detail of the burner of FIGURE 1.

FIGURE 3 illustrates the burner of FIGURE 1 applied to a radiant tube having an eductor at the exhaust end.

FIGURE 4 illustrates a simplified embodiment of a burner embodying the invention.

FIGURE 5 illustrates a development of a portion of FIGURE 2. Q

FIGURE 6 illustrates certain modifications that can be made to the embodiments of FIGS. 1 and 4.

Referring to the drawing, andin particular to FIGURE 3 burner assembly 11 is shown in operable relationship with radiant tube 12 of hairpin or U configuration having a firing leg 12a, the interior of radiant tube 12 being maintained at a slightly sub-atmospheric pressure by virtue of conventional draft producing means such as eductor 13 or an'exhaust-fan. Radiant tube l2is utiliz ed'to deliver heat to a furnace chamber, which is defined partially be refractory walls 14" and 14a which in practice often'range-from 9 to 13 /2 inches thick. Faces 9 and 9a of walls 14 and 14a are exposed to the full force of the furnace heat and are commonly denoted as hot faces. It' is to beunderstood, that'our invention is not to be limited byother details of furnace 'construction, the particular draft producing means selected, or the configuration ofthe radiant tube which-may be of singleass, U, W'or any other appropriate configuration.

In the embodiment illustrated in FIGURES 1 and 2, burnerassembly. 11' will be constructed principally of two castings;'burner casting 17 and throat casting 18. It is to be understood, however, that the-burner assembly may also beconstructed by other means such as weldments or stampings. Burner casting 17, which is'illustrated isometrically in FIGURE 2, includes backplateZti and cylindrical wal1'21' forming combustion cup 22.

Throat casting IS'is insertedwithin' port 15 of furnace wall 14 in a manner to circumpose at least a portion of firing leg 12a, with flange portion 23 of casting 18 abutting circurnferentially against casing plate 15. Casting 18 may be retained in position by fastening clips (not shown) attached, to casing plate 15 and adapted to bear against tabe (not shown) which are integrally attached to the periphery of flange 23. To prevent airfrom infiltrating into the furnace through'the junction where casting 1d abuts against casingplate 15 a packing materialize, such as'coiled asbestos rope, may be inserted therebetween. Air infiltration through the annular opening defined by the outside of firing leg 12a and the inside of throatcasting 13 may be prevented by means of rings25 which are similar in appearance and function to conventional internal'combustion engine piston rings. Rings 25 are retained in slots 26 of firing leg 12a and are slidable with respect to casting 18. As described, the junction comprising throat casting l8, rings. 25, and, firing leg 12a provides a substantially gas-tight junction and yet readily allows for linear movement of the radiant tube as is caused by thermal expansion and contraction. Burner casting 17' is mounted in place with annular rim portion 27 abutting against flange portion 23 of throat casting 18. Burner casting 17 may be removably attached to throat castinglS by any appropriate fastening means'such as hold down clips (not shown), adapted to bear against tabs 27a on the periphery of rim 2'7.

Combustion cup 22is' attached to rimportion. 27 by means of a number (preferably four) of radially extendinglegs 28..

A gaseous fuel stream such as natural gas or. coke over gas is delivered to combustion cup22 from a conventional fuel supply source (not shown) through conduit 30. Burner casting 17 includes a fuel conduit extension 31 whereby thefuel is discharged into combustion cup 22 at a point somewhat removed from backplate 20. We have found, in the burners we have experimented with, that the internal diameter of extension 31 ought to be such that a stream of natural gas will pass therethrough, when flowing at the maximum desired rate, at a maximum velocity of from 40'to feet per second. At turned down flow rates this maximum velocity will, of course, decrease proportionately. Further we have :3 found that in burners adapted for use with radiant tubes of a conventional size (approximately 4 to 5 inch internal diameter firing leg) that the length of extension 31 ought to be in the range of a 1.0 to 1.5 multiple of the internal diameter of extension with larger burners tending toward the upper portion of the aforesaid range. A portion of the air required for combustion of the fuel stream is added tangentially to combustion cup 22 as by means of air ports 32 in lobes 33 of burner casting 17. It is essential that air ports 32 be located upstream of the outlet of extension 31 and it is preferred for optimum burner performance, that the air ports 32 be located as nearly adjacent to baclcplate as is possible with the manufacturing techniques employed. We have found that the quantity of air admitted through air ports 32 is desirably from 3 to 7% and preferably from 5 to 7% of the total air required for stoichiometric combus tion of the maximum fuel stream. We have further found that it is not necessary that air be delivered from a compressor to air ports 32. By properly sizing the air ports 32 it is possible to induce the desired quantity of air from the atmosphere to flow thereinto by virtue of the draft within tube 12.

By virtue of its tangential admission the primary air stream entering through air inlet ports 32 is caused to spin rapidly about extension 31 thereby creating a vortexeifect suction adjacent point of termination of extension 31. Such suction causes the outside portion of the fuel stream passing from extension 31 to the drawn backwards along the outer surface of extension 31 towards backplate 20. This portion of the fuel stream is caused to mix with the primary air stream thereby forming a combustible mixture adjacent backplate 20. This mixture can be readily ignited, as by placing a torch near lighting port 34. A sparking device may also be used if desired.

A special feature of the tangential admission of primary air through ports 32 to combustion cup 22 is that it prevents quenching of the stable flame front at backplate 20.

Due to normal manufacturing variations, there are apt to be a number of burner castings out of every lot in which air port 32 is not as closely adjacent to backplate 20 as is normally desired. This condition can, in aggravated circumstances, lead to instability of the flame within the combustion cup. We have found, however, that this condition can in many cases be remedied by inserting an annular ring 35 within combustion cup in substantial contact with wall 21, preferably at a point somewhat downstream the end of extension 31. The added turbulence imparted to the fuel and air streams by virtue of ring 35 tends to compensate for the diminished turbulence caused by the improper location of air ports 52 and therefore ring 35 makes the location of air ports 32 somewhat less critical. Also, the location of ports 32 may be made less critical. Also, the location of ports 32 may be made less critial by constructing the ports of oblong rather than circular section.

The flame from the combustion reaction which occurs in combustion cup 22, as described above, is used as a pilot flame to initiate and maintain combustion of that portion of the fuel stream which passes uncornbusted from combustion cup 22. The length of combustion cup 22 must not be so long as to allow for completion of the pilot combustion reaction within combustion cup, that is, at least some portion of the pilot flame should extend beyond the combustion cup at all firing rates.

An additional or secondary combustion air stream is added to radiant tube 12 through atmospherically exposed annular opening 36 which is defined by combustion cup 22 and firing leg 12a and which may be considered to serve as an air metering orifice. It is to be understood that an equivalent annular opening could be defined by combustion cup 22 and any effective structural extension of firing leg 12a such as surface 13a of throat casting 18. This secondary air stream is circumposed about the fuel stream which issues from combustion cup 22, it being noted that this fuel stream will be heterogeneously mixed with the combustion products of the previously described stabilization combustion reaction.

By virtue of the fact that the fuel stream issuing from the combustion cup is somewhat turbulent and somewhat pre-heated as a result of the stabilization combustion in the combustion cup there is a tendency for the fuel and secondary air stream to intermix and combustibly react too rapidly, especially in that portion of firing leg 12a which is shielded by refractory wall 14. Even a rela tively small amount of combustion in the shielded portion of the firing leg of a radiant tube will cause a severe problem of overheating which can easily lead to premature tube failure. In our burner we have found that overheating of the firing leg can be effectively reduced by inserting a plurality of turning vanes 37 in communication with opening 36.

Turning vanes 37 impart a rapid spinning action to the secondary air stream entering annular opening 36. The secondary air stream set spinning in this manner tends to remain separate from the partially combusted fuel stream which it surrounds for a longer period of time than it otherwise would, thereby decreasing the rate of combustion and heat release in the firing leg 12a of radiant tube 12. Also, the spinning of the secondary air stream tends to positively wipe off heat from the Wall of the shielded portion of firing leg 12a by virtue of the fact that air velocity with respect to the tube wall (and hence convection heat transfer coeflicient) is substantially increased. We have discovered, in the burners We have experimented with to date, that it is important that the angle awhich turning vanes 37 form with respect to the axis of combustion cup 22 as is shown in FIG. 5, not be too large nor too small. hVe have found that vane angles less than 30 impart an insufficient degree of spin to combat the problem of overheating in the firing leg whereas vane angles in excess of 60 cause a vortex to form adjacent the outlet of combustion cup 22 which vortex draws the stabilization flame away from backplate 20, thereby leading to instability of combustion which may be characterized by flame pulsations which lead in turn, to undesirable vibrations and noise.

In addition to the importance of providing the correct v-ane angle or it is also important to properly space each vane from the next adjoining vanes. As is shown in FIG. 5, each vane will have an axially projected length a. Between two adjacent vanes there will be an axially projected space b which is the distance from the leading edge 37a of one vane to the trailing edge 37b of the next vane. Itis import-ant that adjacent vanes be offset from each other, i.e. they must not be overlapping. In other words b must not be coextensive with any portion adjacent dimension a. Preferably, dimension 12 should be about onequa-rter of the value of dimension a. We have found that an overlapping vane construction is undesirable because it leads to the problem of flame flash-back in the vanes, which perhaps is caused by the phenomenon of vortex shedding at the leading edge of the vanes.

As we have mentioned before, annular opening 36 serves .as a meter-ing orifice to meter the proper quantity of secondary air to radiant tube 12, its effect as an orilice being influenced by the number, configuration, spacing and angle of vanes 37 which are inserted therein. It is important that this orifice be liberally sized so that the pressure drop of the secondary air stream passing therethrough not be too great. We have found that the pressure drop of the air stream across opening 36 ought not to exceed 1.1 inches water column at the maximum air flow rate. Higher pressure drops tend to cause flashing of the flame back into the vanes and tends also to draw the stabilizing flame away from the b-ackplate 20 of comsubstantially the same in all respects as the embodiment of FIGU'R ES 1 and 2 except in its mode of attachment to-the radiant tube, Theburner ofFIGURE 4 is constructed of only a single casing, i.e. burner, casting 117. Casting 117 is mounted at least partially. within modified. firing leg 112a of radiant tube 12 and is secured in place by means of screw 41 which seatsin port 42 located in cylindrical Wall 121. Firing leg 1 1'2atis secured to burner mounting plate 43 by means of a we1d44 which is continuous to retard air from infiltrating into the furnace chamber. Plate 43- is in turn secured to casing plate 1 5 by means of weld.45 whichi-s also continuous to retard air infiltration.

One ofthe outstandingfeatures of this invention, a feature common'both to the embodiment-sci FIGURE 1. and [FIGURE 4, is that the burner unit is mounted at a point substantially removed from the hot face 9 of furnace wall. The simplification of burner structure that can be attained by mounting the burner so remotely is self-evident when this burner is compared to prior artburners such as Cipriani et al., 2,843,107; Munford, 2,051,699; and Parker et a1. Patents 2,796,118 and 2,820,447. However, the problem of overheating in the shielded portion of a radiant tubefiring l-eg has, heretofore, been overcome only by structurally impeding combustion in this region as the aforesaid patents illustrate. The elimination of the structureformally required to impede combustion in this region, which is made possible by this invention, not only permits simplification of the burner construction but also substantially eliminates the maintenance problems associated with the more complicated prior art burners. Various modifications can'bemade to either the burner embodiment of FIGURE 1 or. the embodiment of FIG- URE 4. Several of these modifications are illustrated'in FIGURE 6 in connection with burner casting 17a which is similar in construction to Iburner casting 17. It is to be understood, however, that these, modifications arealso applicable to a burner casting similar to burner casting 1 1'7.

A modification lies in the addition ofradial ports 61 in the wall 21a of combustion cup 22a. Ports 6 1 allow additional air to enter combustion cup and partially mix with the partially combusted air flowing therethrough. This additional air will tend to' increase the rate of combustion of the partially combusted fuel stream and will,

therefore, tend to increase the heat release rate of the associated radiant tube. This expedient may be especially desirable in larger burners (over 5 d-iameterfiring leg) where the phenomenon of carbon drop-out at the tube axis, a phenomenon characteristic of laminar flow diffusion'cornbustion, maybe troublesome.

Another modification lies'in the addition of a restric tion 62 tothe outlet of extension 3 1a. Restriction 62 serves to provide a positive pressure drop at extension 31a which makes it possible to; attain'uniform fuel flow distribution to several burners from a common fuel supply. When restriction 62 is provided it is advisable to provide radial ports 63 in extension 31a upstream of, restriction 6 2; Radial ports 63 permit fuel to escape radially from extension 31a and serve to offset the reduction in fuelflowing' to the backplate caused by restriction 62.

Various other modifications can be made in the apparatus illustrated anddescribed without departing from the spirit of the claims, as the embodiments ofthe invention shown and described are intended as illustrative only.

We claim:

1. Thefmethod of initiatingand stabilizing the combustion of a gaseous fuel stream within a heating tube having, at itsinl'et end a combustion cup comprised of a backplate and a cylindrical wall, said combustion cup being in; fluid communication with said heating tube; which comprises passing a fuel stream axially throughthe backplateand into said combustion cup and discharging said fuel stream within said combustion cupat a point intermediate the ends thereof; passing afirst air stream, comprising 3 to 7% of the total air required for stoichiometric combustion of the fuel stream, tangentially into said combustion cup and discharging the first air stream within thecombustion cup at a point upstream of the point where the fuel streamtis discharged to cause said first air stream to swirl about said fuel stream to create a region of reduced pressure at the point where the fuel stream is discharged and to divert a portion of the fuel stream toward said bachplate by virtue of the reduced pressure created by said first air, stream to form a combustible mixturewith a portion of said first air stream; ignitingsaid mixture at a point adjacent said backplate to create a stablefiame front within said combustion cup.

at a point very closely adjacent said backplate; and passing an annular second air stream into said heating tube to support the combustion of the remaining portion of the fuel stream.

2. The method according to claim 1 which further comprises the step of imparting a high degree of spin to the annular second air stream.

3. The method according to claim 1. wherein the first .air stream comprises 5 to 7%. of the total air required'for stoichiometric combustion of the'fuel stream.

4. Self-stabilizing burner apparatus comprising, incombination: wall means forming a combustion cup, said wall means comprising a cylindrical wall having acorresponding inside and outside surface and the ends of which cylindrical wall define. an inlet and .an exit end of the combustion cup, and a backplateperipherallyconnected to the cylindrical wall at the inlet end thereof; a fuel delivery pipe extending through the backplate. into said cup substantially along the axis'of said cylindrical wall and terminating at a point intermediate said inlet and exit ends of said combustion cup for introducing a stream of fuel substantially axial to said combustion cup; primary air inlet means connected to the outside surface of said cylindrical wall intermediate said backplate and the end of said fuel delivery pipe in a manner for introducing a stream of primary air, comprising 3'7% of the total air required for stoichiometric combustion ofthe fuel stream, said primary inlet means introducing the air tangential to the inside surface of the cylindrical wall whereby a region of reduced pressure is established upstream of the point at which the fuel delivery pipe terminates which causes a'portion of the fuel stream to flow toward said ba'ckplate to form an ignitable mixture with 'said primary air stream which, when ignited. forms a stable flame front between said backplate and the point at which the fuel delivery pipe terminates; means for causing an annular stream of secondary air surrounding the outside surface of the cylindrical wall to flow past the exit end of the combustion cup; and a plurality of spin vanes connected to the outside surface of the-cylindrical wall adjacent the exit end of the combustion cup, the vanes being located in a skew position with respect to the axis of the combustion cup whereby said secondary air stream is caused to spin.

5. Apparatus according to claim 4 wherein the axis of each vane is removed by an. angle of from 30 to 60 (References on following page) 9 10 References Cited by the Examiner 2,796,118 6/57 iarker et a1. 2- 126-91 2,839,123 6/58 Schweitzer et a1. 158-7 ggi liflj PATENTS 158 109 X 2,843,107 7/58 Cipriani etal 158115 X r 2,9'2,307 9 60 1 11/22 Willcox 158-109 5 J Schramm et a1 58 7 1/32 Branche 158-99 FOREIGN PATENTS 5/32 Erickson 158109 X 350,051 6/31 Great Britain. 6/32 Foster 1587 X 8/35 Barthel et a1 15811 X JAMES W. WESTHAVER, Primary Examiner. 10/35 Hepburn 158-99 12/42 Wood 158 15 10 PERCY L. PATRICK, Examiner. 

1. THE METHOD OF INITIATING AND STABILIZING THE COMBUSTION OF A GASEOUS FUEL STREAM WITHIN A HEATING TUBE HAVING AT ITS INLET END A COMBUSTION CUP COMPRISED OF A BACKPLATE AND A CYLINDRICAL WALL, SAID COMBUSTION CUP BEING IN FLUID COMMUNICATION WITH SAID HEATING TUBE; WHICH COMPRISES PASSING A FUEL STREAM AXIALLY THROUGH THE BACKPLATE AND INTO SAID COMBUSTION CUP AND DISCHARGING SAID FUEL STREAM WITHIN SAID COMBUSTION CUP AT A POINT INTERMEDIATE THE ENDS THEREOF; PASSING A FIRST AIR STREAM, COMPRISING 3 TO 7% OF THE TOTAL AIR REQUIRED FOR STOICHIOMETRIC COMBUSTION OF THE FUEL STREAM, TANGENTIALLY INTO SAID COMBUSTION CUP AND DISCHARGING THE FIRST AIR STREAM WITHIN THE COMBUSTION CUP AT A POINT UPSTREAM OF THE POINT WHERE THE FUEL STREAM IS DISCHARGED TO CAUSE SAID FIRST AIR STREAM TO SWIRL ABOUT SAID FUEL STREAM TO CREATE A REGION OF REDUCED PRESSURE AT THE POINT WHERE THE FUEL STREAM IS DISCHARGED AND TO DIVERT A PORTION OF THE FUEL STREAM TOWARD SAID BACKPLATE BY VIRTUE OF THE REDUCED PRESSURE CREATED BY SAID FIRST AIR STREAM TO FORM A COMBUSTIBLE MIXTURE WITH A PORTION OF SAID FIRST AIR STREAM; IGNITING SAID MICTURE AT A POINT ADJACENT SAID BACKPLATE TO CREATE A STABLE FLAME FRONT WITHIN SAID COMBUSTION CUP AT A POINT VERY CLOSELY ADJACENT SAID BACKPLATE; AND PASSING AN ANNULAR SECOND AIR STREAM INTO SAID HEATING TUBE TO SUPPORT THE COMBUSTION OF THE REMAINING PORTION OF THE FUEL STREAM. 